Electronic devices consume a great deal of energy during normal operation. This energy is transformed into heat that is concentrated within devices that are generally sealed. With the concentration of integrated circuits, the electronics of a modern device release more heat per unit of surface than older devices. This concentration leads to a temperature increase which is eventually prejudicial to the reliability of the integrated circuits and other sensitive elements such as central processing units or the hard disk. To resolve this problem, a cooling system is implemented into the electronic devices using one or several fans which blow cool air from the exterior in order to cool the interior. The integration of a hard disk into television decoders, thus enabling the functions of a PVR (Personal Video Recorder), considerably increases the heat released by the set of active circuits. For example, it is not rare to find on the market dual reception High Definition PVR products having an average consumption in the order of 40 W.
If it is not evacuated, this heat provokes an accelerated aging of the electronic components that results in an irreversible deterioration. To avoid this, the manufacturers cut air inlets in the device box to enable the evacuation of the hot hair. A fan is positioned in front of the air inlets to accelerate the exchanges with the exterior and allow correct internal cooling. But using a ventilator at full speed is noisy. If the device is a television decoder which can, for example, be located in a bedroom, in this case, it must not disturb the users sleep. The maximum threshold usually authorized is twenty five decibels. This measure is realized according to the Sound Pressure method, i.e. using a microphone positioned at one meter from the front panel of the decoder.
To reduce the sound level, the servo system of the fan checks regularly the speed and modulates it according to a directive to be attained. A temperature sensor is positioned in an area of the device that is usually hot. As soon as a temperature threshold is reached or exceeded, the fan speed increases proportionally to the difference between the temperature threshold and the value measured by the sensor. During switching on of the device, the fan is turned on at slow speed, and as soon as the temperature exceeds 50° C. inside the device, the fan speed is increased. If a certain rotation speed is exceeded, the device becomes noisy. To control its operation, the fan is fitted with a tachymetric sensor which transmits a signal according to the real speed of the blades. The control unit of the fan analyses the signals coming from the tachymetric sensor and ensures regulation of the fan speed. The regulation is carried out by a of PDI (P for proportional, D for differential and I for integral) type automation. This efficient automation enables precise regulation of the speed, by adjusting the command supplied to the fan according to a directive calculated from values measured by the tachymetric sensor.
The electronic components experiencing difficulties in supporting excessive heat, the designers are confronted with the following dilemma: either to privilege the absence of noise and risk an overheating of the components, or to ventilate the interior of the device as well as possible but rendering it noisy. The present invention enables among other things the resolution of the dilemma, at least under certain circumstances.
Recently, the designers of electronic devices have discovered that by positioning a device vertically, the presence of air inlets at the top and bottom of the structure enables an increased level of natural convection inside the device, as for a fire in a fireplace. This fireplace effect considerably increases the natural cooling of the device. The present invention takes into account this convection phenomenon and takes advantage of it to minimize the noise produced by the device.
The document US 2007/296408—LIAO TS-HSINE describes a system for controlling temperature using electronic sensors and a system for controlling a plurality of fans. The sensors are spread over the printed circuit and then provide a mapping of the heat dissipated. Each value is compared with a threshold value associated with the area where the sensor is located, and if the value is exceeded, one or several fans are enabled (to cool the area considered).
The document US 2006/155424—KATOH TAKAYUKI describes an air-cooling mechanism for a mobile PC. A fan is used to cool the interior. An accelerometer detects a displacement of the PC, which changes its operating state. In this new operating state, the processor carries out a measurement of the temperature and possibly changes the fan speed or the internal clock of the PC.
The document U.S. Pat. No. 6,542,846—MILLER describes the temperature regulation of a device which comprises batteries. Some sensors measure the heat released by the batteries and according to the measurement, determine if it is necessary to change the cooling-fan control. Several fans can be used to cool specific areas.
All these documents relate a speed regulation based on the temperature sensor, and thus the activation of the fan whatever the position of the devices described in these documents. Hence, on being powered on, the devices are noisy though it is perhaps unnecessary.